1. Field of the Invention
The present invention relates to a laser drive apparatus and an image forming apparatus.
2. Description of the Related Art
In electrophotographic-type image forming apparatuses, an edge emitting laser (EEL) and a surface emitting laser (SEL) are often adopted as light sources. In particular, a vertical cavity surface emitting laser (VCSEL), which is one type of surface emitting lasers (SEL), draws attention. In a VCSEL, a plurality of light-emitting points can be provided on a semiconductor substrate, so that an electrostatic latent image can be effectively formed by using a plurality of laser beams.
Meanwhile, the light amount of a laser beam that is emitted from a semiconductor laser varies depending on the temperature of the semiconductor laser. This may cause density unevenness in an image or the like. Therefore, automatic light amount control (APC) for maintaining constant a light amount of the laser beam is performed (Japanese Patent Laid-Open No. 2002-40350). With APC, a driving current is set that enables the semiconductor laser to emit a laser beam of a constant light amount.
Meanwhile, laser light-emitting elements have the property of emitting light only upon receiving a driving current of at least a certain threshold Th. In other words, a laser light-emitting element does not emit light if an electric current of less than the threshold Th is supplied. In order for the laser light-emitting element to emit pulse light without delay of turn on, it is necessary to constantly supply the laser light-emitting element with a bias current of approximately the threshold Th. However, since this threshold Th as well is temperature dependent, it is also necessary to obtain the threshold Th when performing the APC.
The threshold Th can be obtained as an x intercept of a function f(x) that expresses light amount (y axis) as a function of driving current (x axis). Assuming that this function is a linear function, this function can be determined if two coordinates (I_Pa, Pa) and (I_Pb, Pb) are known. Here, Pa and Pb each denotes a light amount (Pa>Pb). Ordinarily, Pb is a fraction of Pa. Also, I_Pa and I_Pb are the driving currents that correspond to Pa and Pb. I_Pa and I_Pb can be obtained by performing APC with respect to the light amounts Pa and Pb. From (I_Pa, Pa) and (I_Pb, Pb), the threshold Th is obtained and then a bias current is determined. The bias current is ordinarily set to a current that is approximately 90% of the threshold Th.
Meanwhile, in order to correctly obtain a threshold Th, it is required that Pb, which is the relatively lower light amount between Pa and Pb, have a value as small as possible. This is because if Pb is high, an approximation error of the function may become large. This tendency is likely to be noticeable if the temperature of the laser light-emitting element increases. On the other hand, if the light amount Pb is too low, so-called synchronous detection for acquiring a scanning timing of the laser beam cannot be performed.